There are a number of known solutions for measuring the surface speed of a vessel.
A first solution consists in performing navigational tests before the vessel is placed in operational service. These tests consist in measuring the speed over ground of the vessel over a given round-trip in a time interval of short duration, i.e. an interval during which wind and current conditions may be considered to be sufficiently stable. This round-trip, which is performed over a given navigational course, allows the drift of the vessel due to current and wind to be canceled out. In this case, the average speed over ground over the round-trip may be assimilated to the surface speed. This type of operation may be repeated a number of times under various navigational conditions (propulsion regimes). Repetition of this type of operation makes it possible to establish a lookup table between engine regime and surface speed, which may be used by the crew to estimate the surface speed of the vessel under normal navigational conditions. However, the surface speed values given by this solution are approximate under real navigational conditions and, furthermore, this solution requires information be obtained on the engine regime of the vessel, by way of a specific on-board sensor. This method cannot for example be used by a land-based service to simultaneously measure the surface speed of a large number of vessels engaged in operations (i.e. with a load that varies depending on the trip) in a given zone, i.e. vessels neither the loading conditions nor the engine regime of which are known with precision by the service in question.
Another solution consists in using an on-board speed sensor specifically to measure the relative speed of the vessel with respect to the surface of the water. This type of sensor is commonly called a log, and generally provides information on only one component of the surface vector (the longitudinal component corresponding to the projection of the velocity vector onto the axis of the vessel). The surface speed measured using a log is available on board the vessel but a land-based service, for example one in charge of monitoring maritime traffic, does not have access to this information. It may in certain cases be communicated thereto by the vessel using a radio transmission means, but cannot be measured directly on land from the data commonly acquired by this type of monitoring service.
There are also a number of known solutions for measuring environmental parameters such as wind and marine surface current.
Marine-Surface-Current Parameter:
Drifting buoys are one solution for measuring this parameter. These drifting buoys are used to follow the movement of a mass of water (Lagrangian drift) and to measure surface current. They periodically transmit their position via satellite telecommunication means. The drawback of these pieces of equipment is that they are unsuitable for coastal zones, especially because of the high maritime traffic density and the risk that the equipment will run aground. In addition, data are only available in non-real time and the costs of acquisition of the hardware and of deployment are high.
Another solution consists in using high-frequency (HF) or very high frequency (VHF) radars, which allow surface current measurements to be taken in coastal zones in almost-real time. The echoes received as return from the radars are multiple and very varied because of the innumerous types of waves that agitate the surface of the sea. It is known to distinguish and identify waves the wavelength of which is equal to half those emitted by the radar. Since the theoretical propagation speed of these waves in the absence of surface current is perfectly known, it is then possible to deduce therefrom by difference with the measured speed (Doppler shift), the speed of the current. However, it is necessary to have two radars scanning a given zone to reconstruct the vector of the current (magnitude and direction) from the radial components measured by the two radars at a given point. The common measurements of the two radars then allow a map of the surface currents to be charted. The area of the measurement zones remains quite small and the spatial resolution of the measurements is low (about 25 km2), especially when the measurement distance from the radar is large. HF radars allow measurements to be taken up to a distance of 200 km from the coast. This type of equipment is very expensive.
Another solution is measurement from satellite platforms. It is possible to measure surface current using satellite radar images (referred to as synthetic aperture radar or SAR images). This solution allows radial measurements of the current to be obtained over vast areas but with low resolution. It is also possible to use a satellite-mounted altimeter and to measure surface current by interpreting differences in ocean surface level (geostrophic hypothesis). However, this method is limited, because of the small number of satellite altimeters able to provide the most regular possible temporal and spatial coverage and also because the current measurements concern large-scale movements of the entirety of the water column and not the surface current directly. It is also possible to estimate surface current using satellite measurements of the ocean surface temperatures or water color (optical means). These methods are not very effective in the presence of clouds and at the present time do not satisfactorily take account of the dynamics of real surfaces. Generally, measurements taken using satellites are transmitted with a delay, which depends on the telecommunication systems used, between space and earth and the measurements carried out are less precise than those carried out for example using a high-frequency coastal radar.
Another solution consists in using a current meter intended to measure the flow speed of the water at a fixed position (Eulerian measurement of current). This measurement may be taken using buoy-mounted mechanical current meters, or the current meters called acoustic Doppler current profilers (ADCP). The measurements performed using this type of instrument are localized (measurement over large geographical zones is not possible) and the maintenance of instruments deployed at sea is expensive.
Another solution consists in equipping vessels with Doppler current meters, often referred to by their abbreviation VM-ADCPs (for vessel-mounted ADCPs). The vessel-mounted Doppler current meter is a piece of apparatus capable of recording a profile of current velocities and current directions. These data may be transmitted in almost-real time, if the vessel taking the measurements is equipped with suitable telecommunication means. However, few vessels are equipped with this type of expensive measuring equipment, which is furthermore difficult to calibrate. These vessels therefore do not allow continuous measurements to be obtained over vast maritime areas.
Another known solution consists in exploiting the navigational parameters of a vessel (speed over ground, heading, surface speed) to deduce information on the marine currents by analyzing the drift of the vessel. This technique is commonly referred to as dead reckoning and is used to approximately estimate surface current. The position, heading and speed over ground of the vessel are measured at a given instant. A second measurement of the same parameters is taken at another instant, conventionally a few hours afterward. These two measurements of navigational parameters taken at different instants allow surface current (or drift current) to be estimated. A vessel navigating in a zone with no surface current will arrive at a given instant at a position predicted beforehand. In the presence of a current, the vessel will have deviated from its course and will not be at the predicted position. It is then possible to obtain an estimation of the drift current by summing vectorially the course estimated on the basis of the navigational parameters measured at a given instant and the actual course followed by the vessel. However, this technique does not allow a precise measurement of surface currents to be taken in real time since it especially requires a posteriori knowledge of the course actually followed by the vessel.
Marine-Wind Parameter:
Ocean surface winds may be measured by means analogous to those described for measuring surface current. One solution consists in using a buoy-mounted anemometer and weathervane. Another solution consists in using radar measurements, the radars possibly being land- or satellite-based (measurement of wind by scatterometry or SAR imagery). These solutions suffer from the same drawbacks as those mentioned above with regard to current measurement, namely high maintenance and implementation costs, low spatial coverage when the solution is able to provide a high spatial and temporal resolution (case of wind measurements at fixed points) or, conversely, a large spatial coverage but with a low spatial and temporal resolution (case of satellite means).
More generally, the use of environmental parameters such as wind and current to optimize the course of a vessel is widespread. Document US2012/0259489 for example describes a system for automatically piloting a vessel allowing its course to be optimized by taking into account environmental parameters such as wind and current measured using vessel-mounted sensors. The optimization system presented consists in adapting the surface speed of the vessel (i.e. its engine regime) to follow a planned (i.e. set, but not actual) course with respect to the seabed. The described system requires measurements to be taken of environmental parameters such as wind and current, at the position of the vessel, using sensors specifically mounted on board. It is also necessary to know the surface speed of the vessel, determined using another on-board sensor dedicated to this purpose and directly connected to the engine block. Document EP 0319395 also describes a system for controlling or assisting with maritime navigation exploiting environmental data such as wind and current. Just like the aforementioned system, this document describes a system that requires a speed sensor to be mounted on board the vessel to measure the surface speed of the vessel, i.e. a speed measurement that may be used to deduce a measurement of drift current.
These two documents also mention the use of statistical databases (such as current atlases) or forecast models, i.e. data that are by nature different from actual measurements. These data do not have the precision required to determine the actual surface speed of the vessel in question at a given instant or the actual drift current in proximity thereto and for this reason these documents also describe the use of additional measurements carried out using on-board sensors, to mitigate this difficulty.
The aim of the present invention is to palliate these drawbacks and to provide a method that, while nonetheless being simple to implement, allows the surface speed of a vessel to be calculated and wind and marine surface currents to be measured.